How to get
This powerful collar encased in a fiery orb has a strong and magical hold on one particular mouse in The Marching Flame: the Gargantuamouse!
Arm this charm when you've reached a streak of at least 7 catches in the Fiery Warpath. If your cheese attracts a mouse, this charm will ensure it is a Gargantuamouse. Just be ready to face this draconic beast...
After catching the mouse, the streak will be broken and you should disarm this charm!
Mousehunt Wiki page